One of the best indoor exercises for abdominal area of the body is sit-ups. It is also noted that more people are becoming interested in exercising regularly. Sit-ups can be done with one person holding another person's feet or sticking the feet under the bed, a chair, etc. This is hard on the metatarsal part of the foot.
There are several U.S. Pat. Nos. such as 4,212,458 and 4,185,816 using a door as a base support. In both cases these units stand out in the road for possible injuries in the dark or stumbling anytime.
For this reason I feel that the bed frame sit-up exerciser is more efficient because it is out of the way when not in use, stowed under the bed. It can be extended when being used and will self retract under the bed when the feet are removed.